i18n: Look up brushsettings.py translations from "libmypaint" domain
i18n: Look up brushsettings.py translations from "libmypaint" domain

We have to be explicit about the domain since the file is part
of a reusable library now.

achadwick/libmypaint,achadwick/libmypaint,achadwick/libmypaint,achadwick/libmypaint,b3sigma/libmypaint,b3sigma/libmypaint,b3sigma/libmypaint